1. Field of the Invention
The present invention relates to a fluid-filled bushing, and more particularly to a fluid-filled bushing with variable damping forces which is to be disposed between a suspension member of a vehicle and a frame of the vehicle.
2. Description of the Relevant Art
Generally, vibro-isolating rubber bushings capable of damping vibration are disposed between suspension members of vehicles and vehicle frames on which the suspension members are pivotally mounted. It is desired that such bushings produce relatively large damping forces in order to provide good vehicle maneuverability, an ability for the vehicle to run straight at high speed, and to keep the vehicle stable in its attitude when it is started or stopped. However, from the standpoint of providing good riding comfort and preventing vibration and noise from being trasnsmitted from wheels, the bushings are required to have relatively small damping forces.
Japanese Laid-Open Patent Publication No. 59-116649 discloses a bushing with variable damping forces which meets such contradictory requirements. The disclosed bushing is mounted on a frame-supported portion of a suspension arm which supports a wheel for movement in the longitudinal direction of a vehicle frame, and comprises an inner tube coupled to the vehicle frame, two outer tubes connected to an end of the suspension arm, and a body of vibro-isolating rubber interconnecting the inner and outer tubes. The rubber body has two fluid chambers defined therein in circumferentially spaced relation, and the fluid chambers can communicate with each other through an annular passage defined between the two outer tubes. The fluid chambers and annular passage are filled with a fluid. The annular passage is selectively opened and closed by a valve body which is actuated by an actuator mounted on the outer periphery of the radially outward outer tube, for movement into and out of the annular passage. The damping force produced by the bushing is varied by such selective opening and closing of the annular passage. In the above fluid-filled bushing with variable damping forces, the valve body actuator projects outwardly from the outer tube. Therefore, an installation space for the actuator has to be provided on the bushing, and a special measure must be taken to protect the actuator against external forces.